In CSPs of the type having a central tower, a large number of heliostats (in the form of planar mirrors) reflect the solar light toward one or more solar receivers, situated at the apex of the tower, the heliostats being positioned such that the shadows created by the mirrors do not interfere with the adjacent mirrors.
The solar receiver, heated by the concentrated incident solar rays, will generate a hot fluid that will be next used at ground level to produce high-pressure steam capable of driving a turbine and of producing electricity.
The fluid heated at the apex of the tower can directly be steam, or air, or a thermal oil. However, it may also be a molten salt consisting of a mixture of two or three, or even more, specific salts used as thermal-transfer fluid.
For example, a mixture of sodium nitrate (NaNO3) and potassium nitrate (KNO3) is often used, for example at a 60%/40% ratio, forming an atmospheric-pressure eutectic with a melting temperature reduced to 220° C. and offering good chemical and thermal stability between the melting temperature and 600° C. By using a ternary mixture of salts, comprising lithium nitrate (LiNO3) in addition to the two aforementioned salts, it is even possible to obtain a eutectic having a melting temperature as low as 120° C.
One major advantage of this mixture of salts is the possibility to store it in large quantities at high temperature and atmospheric pressure, at a reduced cost. The storage allows to separate the capture of solar energy and the production of electricity, independently from sunshine and solar hour, including at night.
The operating principle of a combined-cycle CSP power plant is known and for example described in document WO 2011/077248.
FIG. 1 diagrammatically shows the principle of a concentrated solar power plant of the tower type 1. The salt is maintained liquid in a first, insulated cold storage reservoir 2, at a temperature that is not lower than 260° C. Pumps 3 are necessary to bring the molten salt to the apex of the tower 1, and given the high flow rates required and the high density of the salt, the power absorbed by the pumps is relatively high, of an amplitude of 4 MW for a high-power power plant (typically 150 MW). At the apex of the tower, the salt is heated to 550° C. by the concentrated solar heat as specified above by means of one or more exchangers 20, distributed for example in four cavities, made up of thin-walled steel tubes. From there, the heated salt is returned to a second, insulated hot storage reservoir 5. The capacity of this reservoir depends on the supply duration required for the turbine that produces the electricity. When the production of electricity by the power plant is required, the hot salt is sent by a pump 6 to a conventional steam-generation system 7 to produce overheated steam for an electricity generator 9 having a turbine 8. FIG. 1 further shows a detailed example that is non-limiting with respect to the present invention, of a steam generator 7 according to the state of the art.
The molten salt circuit is referenced 17 and the water/steam circuit is referenced 18 in FIG. 1.
Standard performance levels for a 150 MW installation are provided in Table 1.
It is also known that hydraulic power recovery turbines 4 (HRPT) could be used in this type of installation. These may be installed in the line for returning the heated salt to the storage reservoir, in order to recover the mechanical (gravitational) energy from the salt descending from the apex of the tower to the ground, the recoverable power having a typical amplitude of 3 MW for the aforementioned power plant.
In addition to a certain number of advantages, such as large storage capacity for energy at atmospheric pressure, low cost of the salt compatible with environmental safety, complete lack of fire risk, great simplicity and reduced costs for the solar receiver and associated equipment at the apex of the tower, CSP power plants with towers have several drawbacks, including the need to use very specific pumps, the design of molten salt/water-steam exchangers and the need to monitor the relatively high temperatures of the molten salts.
Document WO 2011/018814 discloses a method for locally pressurizing a first circuit in which a first heated fluid at a first pressure flows, and for providing that first fluid to a heat exchanger in order to exchange heat with a second fluid flowing in a second circuit at a second pressure that is greater than the first pressure. A pressurizing means, such as a pump, is provided in the first circuit to increase the pressure of the first fluid upstream from the inlet of the exchanger to a pressure corresponding to that of the second fluid. On the return line of the first circuit, a pressure-reducing means is provided, such as a butterfly valve, to decrease the pressure of the first fluid downstream from the outlet of the exchanger. A hydraulic motor comprising a turbine or a centrifuge pump used as a turbine is inserted downstream from the butterfly valve. The hydraulic motor and the pressurizing pump are connected to a same variable-speed electric motor working on the same shaft. Thus, the hydraulic motor actuated by the stream of pressurized fluid returning from the heat exchanger not only lowers the pressure of the fluid itself, but further provides the power necessary to operate the pressurizing pump, which consequently reduces the external electricity contribution.